JimmyL you crack me up. :D

What your mechanic did is BAD. I'd put that back into working order right away. That is there to prevent overboost fueling and damage to the engine in the case of a waste gate failure on the turbo. Your "mechanic" doesn't know what he's doing! (Or he does, and thats even worse!)

Mine has been, (CD) I don't know for how long before me, but I've owned it for six years. An MB mechanic spotted it and mentioned that's not the first one bypassed. He wasn't concerned about it. This came from someone that usually will explain you need something that you don't.

:Ddan u are the man,cleaned out the hose and valve and didnt get much debris out,the sender looking thingy had alot of carbon on it... but what a difference,forgot how much power the car had ...ty again sir

Adjust your linkage yet?

For general consideration, here's what Stu Ritter and Richard Easley had to say about this bypass issue all those years ago in a FAQ on the old Mercedes-Benz Discussion List hosted on some Baylor U server, which many of you may remember (I assume there are lots of connections between that list and this forum):

"An important point to remember regarding the overboost sensor switch is that there is a 'reverse' release or venting that takes place which is important. When the car is decelerating, you still have a lot of boost which is not needed. During this time, you don't need the accompanying fuel that would normally be injected at a given boost level (due to receiving 'false' information on boost 'needs'). The overboost sensor switch vents the boost at this point so that the injection pump is not getting the signal to continue fuel flow at that particular boost level. This is specifically noted here since it has been noted that these switches are bypassed by unknowing do-it-yourselfers who, in addition to causing the engine to consume excess fuel during deceleration, put their engines in jeopardy by removing this catastrophic failure device."

We can probably safely assume they meant "catastrophic failure prevention device," but the point is well taken.

I call bs on the W123 for several reasons.

1. It's not listed anywhere in the FSM as having that functionality.
2. Pre 85/cali 84 there is no way the system can know that.
3. As soon as you let your foot off the pedal the boost disappears.
4. No one has had a problem with it yet.

Sounds like someone was applying gasser technology to diesels......

Easley also recommends the bubble method for timing which is horribly inaccurate. Of course some of this other write ups are priceless.

Not that I recommend by passing it but there are plenty of people who are running no waste gate hose at all, including myself once on accident. I realized this on a 300mile trip when the car seems to be laboring more than normal. These pumps just don't put out enough fuel.

So there is no downside at all to bypassing that switch, it's perfectly safe?

Ideally it should be left in place....the engineers wouldn't have put it there if it served no purpose.

Kinda my sense of order too. Thanks.

I'd still like to know, though, why it is that in a "Pre 85/cali 84 there is no way the system can know that," and "As soon as you let your foot off the pedal the boost disappears," as winmutt put it. Not that I'm doubting these are true; I'd just like to know.

Put a boost gauge on your car and see for yourself.
Only the 85 all/84 cali cars had rack and linkage position senors, I don't see any other way for easley's claim to be true. How would the car know it was slowing, electronically?

Sounds very familiar to me too, with my 85. The cleaning of the banjo bolt, etc., made a noticable difference.

However, the real blast came after following advice from Brian Carlton elsewhere on this Forum (in the "It's CRITICAL" thread, I believe, at to plug the vacuum supply to the EGR and the Air Recirculating Valve (approx name?) there at the front of the turbo. Prior to this, the car felt like it made boost way too late and dumped it way too early, taking forever to build off the line and kinda narrating the whole affair with a lot of whistling, as if to mock you about how much turbo functionality you were just venting into the wind.

In my case, the boost-wasting villain is the ARV valve. Originally, I disconnected the vacuum lines to the ARV and the ERG both, and stuck golf tees into the rubber elbows coming out of them. Then I test drove the car. I mean, then I test drove the NEW car. Wow. Not only was the acceleration dramatically improved, the shifting now churned butter-smooth and on time, especially the 1-2 shift. And the mocking whistle stopped too!

After further experimentation I determined the difference-maker was the ARV valve. Plugging both made the improvements. Reconnecting the EGR and plugging only the ARV made the same improvements. Reconnecting the ARV and plugging only the EGR made no improvements. Reconnecting both obviously left it in its original pitiful state, no improvements.

So you might try this little procedure. Nothing like getting a new world of performance with two golf tees! Use your favorite colors, get fancy!

For longer-term solutions, Carlton et al. fabricated some blocking plate kits some years ago, per the http://www.peachparts.com/shopforum/diesel-discussion/92534-egr-blocking-plate-kit.html?highlight=egr+kit thread. Does anyone know whether these are still around/available?

Also, I understand the ARV ultimately functions to signal the EGR, so is it possible the EGR is still what's falling down?

Are there any downsides to completely disabling the ARV and the EGR?

Thanks gentlemens.

Said engineers need to justify their existence.

The way a turbo works is to take exhaust gases and use them to spin an impeller that draws air into the engine. As soon as you let off the gas, there is much less exhaust flowing through the turbo, and nothing causing it to spin. If you suddenly lift, you can hear the turbo spool down. Likewise, the turbo won't spool up until a certain RPM--it is about 1500-2000 in our cars (My tach is broken, sorry if this isn't exact:P) because there aren't enough exhaust gases flowing through the thing.

Thanks man. The tentacles of the vacuum system on this car are so very, um, impressive, you could tell me just about anything is affected by it and I’ll believe it. Even to the point of mystifying basic mechanics, such as these of the turbo you describe. I guess I thought the boost somehow got magically translated as misleading vacuum to the IP, ultimately (via the line from back of the intake to the firewall switch to the ALDA). But I see the simple point you and winmutt make that this is kinda a mechanical impossibility.

Again, though, tell me the condition of the veneer on the wood trim is controlled by vacuum and I’ll reach for the MityVac and look for a line to test.

Anyway, thanks again. Perhaps the overboost sensor switch should be renamed after the great Kinks song about “…the destroyaaah.” The “overboost paranoia switch?”